1. Field of the Invention
The present invention generally relates to a semiconductor memory device and methods for inspecting and manufacturing the same. More particularly, the invention relates to a semiconductor memory device wherein any word line short-circuited with a bit line may be replaced by a normal word line, as well as to methods for inspecting and manufacturing the memory device.
2. Description of the Background Art
As disclosed in Japanese Patent Application Laid-open No. Hei-8-180699, there are known semiconductor memory devices in which any bit line short-circuited with a word line may be replaced by a normal bit line for recovery from the defect. In such former semiconductor memory devices, when standby state is implemented, i.e. , when no data is required to be written to or read from memory cells, bit lines carry a predetermined potential Vcc/2 as well as word lines are brought into conduction with a ground potential Vss.
If a bit line and a word line are short-circuited in the standby state, a leak current flows from the bit line to the word line. In that state, the bit line carries apotential whose level is lower than the potential Vcc/2. During manufacture of former semiconductor memory devices, a check is made to see if the bit line potential stays as high as the required level Vcc/2. If the potential of a given bit line is not found to be high enough, a defect is suspected of that bit line. Once a faulty bit line is identified in a semiconductor memory device during its manufacturing process, that bit line is replaced by a redundant circuit previously furnished in the memory device in question. In this manner, a short-circuit that may occur between a word line and a bit line in the semiconductor memory device is circumvented so that all bit lines will function normally in the device.
In the event of a short-circuit between a word line and a bit line in the former semiconductor memory device, a boosting voltage fed to the word lines leaks into the faulty bit line. This prevents the word line potential from being raised normally. Meanwhile, word line driving circuits are generally given greater capability than bit line driving circuits in the semiconductor memory device. Thus even if there is a short-circuit between a word line andabit line, the word line potential tends to be maintained at a level high enough to keep the memory device functioning. In the former semiconductor memory device, the short-circuited word line, therefore, is kept in use without being replaced by a redundant circuit.
However, if a bit line and a word line are short-circuited, the potential of the word line being boosted has little margin for letting the memory device function properly. Where the semiconductor memory device is used in long operating cycles, the word line potential can become so low as to trigger malfunction. In the face of such an eventuality, it is preferred that a bit line and a word line being short-circuited be replaced altogether by redundant circuits.
In former semiconductor memory devices, the presence of a short-circuit fails to affect the word line potential as severely as the bit line potential. This has made it difficult during the manufacture of former semiconductor memory devices to identify any word line being short-circuited therein with a bit line. Hence the difficulty arises of replacing the short-circuited word line by a redundant circuit during the semiconductor memory device manufacture.